• Welcome to BirdForum, the internet's largest birding community with thousands of members from all over the world. The forums are dedicated to wild birds, birding, binoculars and equipment and all that goes with it.

    Please register for an account to take part in the discussions in the forum, post your pictures in the gallery and more.
ZEISS DTI thermal imaging cameras. For more discoveries at night, and during the day.

Eye relief changes with different lighting? (1 Viewer)

justabirdwatcher

Well-known member
I have a question for the forum experts -

I've noticed on many occasions now that the eye relief seems to be just a hair too much on my bins for bright light conditions, and I'll get the dreaded "kidney bean" if I pull the binoculars in tight against my eyes.

But as the light fades (shadow, overcast, twilight) I don't notice the kidney bean effect anymore to the point that when it's really getting dark, I can pull the bins as tight into my eyes as I want with nothing but a nice round full view.

Anyone care to guess what's happening here?

I've tested this many times and always get the same result. The more light I have the more the eye relief seems to be too much for the eyecups, and vice-versa.
 
I've hesitated in trying to come up with an answer because I've read more than once that the exit pupil is not flat, but not known which way it might curve.

I normally measure ER using two two LED flashlights as the light source. Where the spots exiting the eye lens converge is the ER. I've checked a couple of binoculars at full aperture and stopped down to mimic the eye's pupil response to bright light. In both cases the full aperture ER matched the published value, but when stopped down it was apparently shorter by about 1mm. You might expect the blackouts to be less in bright conditions, not more.

I don't know the answer, but I suspect that when the pupil of the eye is dilated there is just more on axis flexibility in the eye positioning without truncating the field of view, and perhaps less sensitivity to vignetting as well? Happy to be corrected.

David
 
Sounds like a reasonable assumption to me ..... :)

It would be interesting to put some numbers to the change in curvature (and hence ER range required) of the eyeball at the extremes of totally dilated, and stopped down to mimimum by bright light .....



Chosun :gh:
 
This is a side effect of spherical aberration of the exit pupil. It's explained on pages 182-183 of Rutten & van Venrooij "Telescope Optics" using the original 13mm Nagler eyepiece as an example because it has such severe SA of the EP that it can't be used in daylight at all.

The figure below from page 182 shows the principal rays from across the field as they converge behind the eyepiece. The field is fully illuminated only when the eye's pupil is larger than the smallest cross section of rays, in this case 3.5mm. If the eye's pupil is smaller than 3.5mm kidney-beaning will result as rays from the outer part of the field are blocked from entering the eye. The figure is simplified for clarity (it should show ray bundles, not just principal rays) and the text explains things much better than I can.
 

Attachments

  • DSC_0149.jpg
    DSC_0149.jpg
    303.6 KB · Views: 148
Last edited:
I am not sure if this is valid, but I make 7mm and 2.5mm cross sections at the front of the eyeball, differing along the axis by 0.5mm.
 
Hi,

another great book I'd like to read... now that I googled it, up came a full scan on the first page of results... although I probably still should get a copy, try to find Harrie on some telestcope meeting in germany or the benelux states and get it signed...

Joachim
 
Hi Henry,

If the eye's pupil is smaller than 3.5mm kidney-beaning will result as rays from the outer part of the field are blocked from entering the eye.

Thanks a lot for the explanation!

I have to admit that I don't quite understand why the effect is not radially symmetric, which seems to be so characteristic that it gave it the "kidney bean" name.

Regards,

Henning
 
I am not sure if this is valid, but I make 7mm and 2.5mm cross sections at the front of the eyeball, differing along the axis by 0.5mm.

Hi Binastro,

I think the figure is for illustration only. I suppose including lines for ray bundles would have led to an unreadable confusion of intersecting lines, but it would have revealed that the situation with this eyepiece is really even worse than shown. The outer field vignetting that causes kidney-beaning actually begins if the eye's pupil is open to less than 6.5mm.

Henry
 
Last edited:
Hi Henry,



Thanks a lot for the explanation!

I have to admit that I don't quite understand why the effect is not radially symmetric, which seems to be so characteristic that it gave it the "kidney bean" name.

Regards,

Henning

It would be radially symmetrical if the eye were perfectly centered and perfectly motionless, which it never is when looking through an eyepiece in the real world. If you move your eye intentionally you'll see the kidney bean flicker around radially from one part of the outer field to another.

Henry
 
Last edited:
Sounds like a reasonable assumption to me ..... :)

It would be interesting to put some numbers to the change in curvature (and hence ER range required) of the eyeball at the extremes of totally dilated, and stopped down to mimimum by bright light .....



Chosun :gh:

CJ,

In light of Henry's post I thought I'd better look again more carefully. I tested a 10x42, 8x42 and 7x36 and used a 20mm aperture mask to stop down the objective. The EP of the 10x would reduce from 4.2mm to 2.0mm, the 8x from 5.25mm to 2.5mm and the 7x from 5.14mm to 2.86mm.

The ER of the 10x42 was 19.8mm and 18.0mm stopped down. The 8x42 was 20.5mm and 19.5 stopped down. The 7x36 was 19.2mm and 18.0mm stopped down. This appears to be opposite to Henry's illustration for that Nagler eyepiece. I know I regularly move the eyecups up or down by about a millimetre on different outings but hadn't particularly related it to light levels. I'll have to wait for some sunshine to try to figure out what's going on. It might be a while. ;)

David
 
Last edited:
Hi David,

Keep in mind that Fig, 16.14 shows only the principal rays of the eyepiece alone without a telescope, so it doesn't include the effect of exit pupil size on the size of ray bundles. Essentially it shows spherical aberration of the exit pupil for an exit pupil with a diameter just above zero. I think for a given eyepiece design the spherical aberration of the exit pupil should improve at smaller exit pupils since a smaller exit pupil would reduce the diameter of the ray bundles and that in turn would reduce the diameter of the smallest cross section of bundles.

Henry
 
This is a side effect of spherical aberration of the exit pupil. It's explained on pages 182-183 of Rutten & van Venrooij "Telescope Optics" using the original 13mm Nagler eyepiece as an example because it has such severe SA of the EP that it can't be used in daylight at all.

The figure below from page 182 shows the principal rays from across the field as they converge behind the eyepiece. The field is fully illuminated only when the eye's pupil is larger than the smallest cross section of rays, in this case 3.5mm. If the eye's pupil is smaller than 3.5mm kidney-beaning will result as rays from the outer part of the field are blocked from entering the eye. The figure is simplified for clarity (it should show ray bundles, not just principal rays) and the text explains things much better than I can.

I think that's the issue. I do appreciate the time you took to research this and respond!
 
Hi Henry,

It would be radially symmetrical if the eye were perfectly centered and perfectly motionless, which it never is when looking through an eyepiece in the real world. If you move your eye intentionally you'll see the kidney bean flicker around radially from one part of the outer field to another.

Ah, that makes sense. (I hadn't personally encountered the effect yet.) Thanks a lot for the additional explanation!

Regards,

Henning
 
Did it again this evening. 1/2 hour after sunset I could pull those eyecups into my eye sockets as firmly as I wanted, and no dreaded kidney bean effect at all. Just nice clear round circles, where only an hour before, I was holding the bins nearly away from my face. Strange.
 
And I want to apologize to the forum members right here and now if I just introduced some kind of unfortunate awareness akin to the "I never saw CA until someone pointed it out to me" phenomenon. LOL Sorry!
 
Hi David,

Keep in mind that Fig, 16.14 shows only the principal rays of the eyepiece alone without a telescope, so it doesn't include the effect of exit pupil size on the size of ray bundles. Essentially it shows spherical aberration of the exit pupil for an exit pupil with a diameter just above zero. I think for a given eyepiece design the spherical aberration of the exit pupil should improve at smaller exit pupils since a smaller exit pupil would reduce the diameter of the ray bundles and that in turn would reduce the diameter of the smallest cross section of bundles.

Henry

Henry,

Sorry I've been slow to respond, domestic issues, but I've now had a chance to add a little detail.

This time I just used the 10x42. The two flashlight were the spaced apart to subtend angles approximately, 1, 2, 3, 4 and 5 degrees to the objective. I then measured the convergence distance for the full aperture and this time a 2mm mask which would result in 4.2mm and 0.2mm EPs respectively.

Unfortunately I haven't found a simple means to illustrate the results as a AFoV ray diagram, but hope the little chart is clear enough. The results surprised me.

Justa....

Unfortunately the weather hasn't been conducive for doing high and low light comparisons, but my strong impression is that with at least one of my binoculars, that is rather finicky for ER, is more susceptible to blackouts in low light than in bright light. Could it depend on the eyepiece design?

David
 

Attachments

  • IMG_20190118_155529.jpg
    IMG_20190118_155529.jpg
    144.9 KB · Views: 28
I was responding to a post that has now been deleted, but I'll leave the amended descriptive parts in case anyone is interested.

If you were to focus on a small light source pointing towards the beam emerge from the eyepiece would be parallel and the diameter of the exit pupil. With a 8x56 that would be 7mm wide. With a 20mm mask that parallel beam should be 2.5mm wide. Altering the angle of the light source to the limit of the field of view will still produce a parallel beam, but now at an angle 8 times that of the angular field of view to the binocular axis. This shouldn't change when you stop the objective down, though the degree of vignetting near the edge will be larger with the larger exit pupil. I don't know if the degree of vignetting might contribute to the apparent positional change?

When we use the sky as the light source instead the full field of view is illuminated and the dimensions of the cone of light leaving the eyepiece should be the same regardless of whether the aperture is 56mm, 42mm, 20mm or 2mm.


David
 
Last edited:
Eye relief is determined by the lenses in the optical train, and is fixed.

There can be ten or a gazillion photons traveling through the instrument and the eye relief (distance from the occular lens surface to the exit pupil) will not change.

(I think)
 
Sorry Henry, that sounds somewhat awry.

If you were to focus on a small light source pointing towards the beam emerge from the eyepiece would be parallel and the diameter of the exit pupil. With your 8x56 that would be 7mm wide. With a 20mm mask that parallel beam should be 2.5mm wide. Altering the angle of the torch to the limit of the field of view will still produce a parallel beam, but now at an angle 8 times that of the angular field of view to the binocular axis. This shouldn't change when you stop the objective down, though the degree of vignetting near the edge will be larger with the larger exit pupil. I don't know if the degree of vignetting might contribute to the apparent positional change?

When we use the sky as the light source instead the full field of view is illuminated and the dimensions of the cone of light leaving the eyepiece should be the same regardless of whether the aperture is 56mm, 42mm, 20mm or 2mm.

Were you using the torch at close range? I got some weird artefacts when I tried it.

David

I came around to having some of the same misgivings about my methods that you expressed, so I deleted my post before you responded. I'll keep playing around with the question over the next few days and see if I can duplicate your measurements.

Henry
 
Warning! This thread is more than 5 years ago old.
It's likely that no further discussion is required, in which case we recommend starting a new thread. If however you feel your response is required you can still do so.

Users who are viewing this thread

Back
Top